It has been widely practiced that an input device is configured with a plurality of circuits in response to a demand for higher resolution to an image input device such as a digital camera, an image scanner in a copier, and an image sensor of a camera which captures from a satellite and an aircraft.
For example, a resolution may be doubled by arranging two rows of CCD sensors with one row being shifted by a half pixel (CCD: Charge Coupled Device). In addition, in a line sensor to obtain a two-dimensional image by scanning using a row of sensors, it is possible to halve the time required for reading by using separate reading circuits for even pixels and odd pixels. As a result, it is possible to double the resolution in the scan direction since the time required for reading one pixel in the scan direction can be halved.
Thus, it is possible to improve resolution by configuring an input device with a plurality of circuits. However, when configuring an input device with a plurality of circuits, a characteristic difference among the circuits becomes a problem.
Even though input/output characteristics of circuits are typically different for each circuit, output pixel values are different for each circuit even when points of the same brightness are captured. Therefore, for example, in a case of an image which is obtained by being captured with different circuits for even pixels and odd pixels, luminance values indicating the brightness of even pixels and odd pixels result in being different. As a result, image quality deteriorates because vertical stripes will be included in the image due to a difference of luminance values between even pixels and odd pixels.
In order to prevent such image quality deterioration, correction is required in consideration of the input-output characteristic of each circuit. In general, when the input-output characteristic of each circuit is known and fixed, it is performed that a value of each pixel is corrected by a correction expression determined in advance.
However, in practice, there is a case in which the circuit characteristic changes for each capturing due to effects of a temperature of the circuit and the like at a time of capturing. When the circuit characteristic changes for each capturing, correction corresponding to an input image is required since the correction expression cannot be obtained in advance.
In PTL 1, an image reading device corresponds to a case that a circuit characteristic changes for each capturing is disclosed. First, before reading image information, a gray scale in which a concentration changes linearly is read and the concentration of the gray scale and an output concentration values of even bits and odd bits at a time of actually reading the concentration are stored respectively for the even bits and the odd bits. Subsequently, image information read by CCD is corrected by proper concentration values for the respective even bits and odd bits. A signal of the corrected even bits and odd bits is composed to be output. In the image reading device in PTL 1, a characteristic difference of linearity of the odd bits and even bits can be eliminated. The image reading device described in PTL 1 is appropriate for applications in which the capturing location is fixed such as an image scanner since concentration correction is performed using a gray scale provided in a reading area.
In PTL 2, an image reading device in which a gray scale for correction in PTL 1 and the like are not required is disclosed. In the image reading device in PTL 2, when a luminance value (pixel data) is smaller than a predetermined value and a difference from an adjacent pixel is greater than a predetermined value, a value obtained by averaging with the adjacent pixel is used and otherwise the original luminance value is used as is. Thereby, a difference between an odd pixel and an even pixel with a small luminance value is eliminated.